Methanol-tolerant cathode catalyst composite for direct methanol fuel cells

ABSTRACT

A direct methanol fuel cell (DMFC) having a methanol fuel supply, oxidant supply, and its membrane electrode assembly (MEA) formed of an anode electrode and a cathode electrode with a membrane therebetween, a methanol oxidation catalyst adjacent the anode electrode and the membrane, an oxidant reduction catalyst adjacent the cathode electrode and the membrane, comprises an oxidant reduction catalyst layer of a platinum-chromium alloy so that oxidation at the cathode of methanol that crosses from the anode through the membrane to the cathode is reduced with a concomitant increase of net electrical potential at the cathode electrode.

RELATED APPLICATION

This application claims the benefit of U.S. patent application Ser. No.10/260,780, filed Sep. 27, 2002.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to direct methanol fuel cells,and, more particularly, to cathode catalyst compositions for directmethanol fuel cells.

BACKGROUND OF THE INVENTION

Fuel cells are considered a possible alternative to direct combustionengines to power transportation vehicles and to possibly furnishelectrical energy to a power distribution grid for home and businessuse. In a fuel cell, fuels are chemically reacted with an oxidantwhereby a direct current is produced at a low voltage across individualcells and stacks of cells produce useful power. Catalyst materialspromote the chemical reactions of the fuels (typically hydrogen ormethanol) and oxidant (typically pure oxygen or air).

In a generic embodiment shown in FIG. 1, a fuel cell 10 includes ananode electrode 14 for the fuel oxidation, a cathode electrode 16 forthe oxidant reduction, and a solid-state polymer electrolyte membrane 18therebetween to provide an ionic conduction path. The combination ofanode electrode 14, cathode electrode 16, and membrane 18 isconventionally called a membrane electrode assembly (MEA) 12. A suitablecatalyst is disposed adjacent the interfaces of electrode/membranesurfaces 14/18 and 16/18 so that the fuel is oxidized at theanode/membrane interface 14/18 to produce ions that traverse themembrane to complete the oxidant reduction at the cathode/membraneinterface 16/18. Fuel 24 is distributed over anode 14 of MEA 12 by fueldistribution plate 22 and unreacted fuel and reaction products 26 areexhausted. Oxidant 32 is distributed over cathode 16 of MEA 12 byoxidant distribution plate 28 and excess oxidant and reaction products34 are exhausted. As a result, electrons generated at anode 14 travelthrough an external circuit (not shown) back to cathode 16. Theelectrons constitute the flow of electrical current that provides energyto components connected to the external circuit.

The most common fuel used in the development of polymer electrolytemembrane fuel cells has been hydrogen, either in a pure form orfurnished as a reformate from hydrocarbon products. Yet another approachis to directly use a liquid methanol solution in direct methanol fuelcells (DMFCs) to avoid the complications associated with supplying purehydrogen or providing a separate system for reforming hydrocarbons toprovide reformed hydrogen. DMFCs with a solid polymer electrolyte canprovide high current density at low temperature and have a relativelysimple fuel cell construction. Methanol is a renewable fuel material andcan be readily transported and supplied with existing transportation anddistribution infrastructure for liquid fuels. Both hydrogen fuel cellsand DMFCs have the generic structure shown in FIG. 1.

In a DMFC, catalysts promote electrode reactions at the cathode and theanode, where a metric of performance is the catalytic activity per unitmass of catalytic metal. This metric is directly related to theefficiency and output power of the cells and to the manufactured cost ofthe cells. Platinum black was an early cathode catalyst in anion-exchange MEA for hydrogen fuel cells, typically a gas diffusionelectrode substrate with one surface coated with platinum black in anamount of 4 to 10 mg cm⁻² of the MEA.

To improve the utilization efficiency of platinum, a catalyst wasdeveloped with a platinum alloy supported on conductive carbon or anunsupported platinum alloy, which was mixed with an ion-exchangepolymer, coated on an electrode substrate, and joined to an ion-exchangemembrane by painting, hot pressing, or the like, to form the MEA. Thisprocess permitted the thickness and composition of the catalyst layer tobe controlled so that the catalyst was more effectively utilized in theelectrode reaction. A loading of platinum or platinum alloy of only 0.1to 1.0 mg cm⁻² was needed to produce a performance equivalent to priorart hydrogen fuel cells.

This reduced loading that has been demonstrated for hydrogen fuel cellshas not been achieved for DMFCs. In a DMFC, some methanol crossesthrough the membrane from the anode and reacts at the cathode, competingwith the oxygen reduction reaction for active catalyst surface sites.Reducing the catalyst loading results in fewer active sites availablefor the oxygen reduction reaction, as well as limiting the ability ofthe catalyst to handle methanol crossover, with a concomitant reductionin the potential of the DMFC cathode. Thus, methanol crossover to thecathode not only lowers fuel utilization, but also adversely affects theoxygen cathode with overall lower cell performance.

One way to reduce the effect of methanol crossover on DMFC performanceis to simply reduce methanol crossover by developing a membrane that isless permeable to methanol. However, this has not yet been fullyrealized in the art. Other ways to reduce methanol crossover includelower methanol feed concentration and optimized cell design.

The present invention recognizes that performance losses associated withmethanol crossover arise from the fact that most Pt-based cathodesystems are catalytically active to methanol oxidation under normal celloperating conditions with a resulting net cathode potential from oxygenreduction reaction potential reduced by the methanol oxidation reaction.In accordance with the present invention, a Pt-alloy catalyst has beenidentified that is less catalytically active for methanol oxidation,while having equal or increased catalytic activity for oxygen reduction.

Various objects, advantages and novel features of the invention will beset forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

The present invention is a direct methanol fuel cell (DMFC) having amethanol fuel supply, oxidant supply, and a membrane electrode assembly(MEA) formed of an anode electrode and a cathode electrode with a solidpolymer electrolyte membrane therebetween, a methanol oxidation catalystadjacent the anode electrode and the membrane, an oxidant reductioncatalyst adjacent the cathode electrode and the membrane, wherein theimprovement comprises an oxidant reduction catalyst layer containingplatinum-chromium so that oxidation at the cathode of methanol thatcrosses from the anode through the membrane to the cathode is reducedwith a concomitant increase of net electrical potential at the cathodeelectrode.

In one embodiment, the platinum-chromium catalyst is supplied at aloading less than 1 mg cm⁻² and the cathode catalyst layer includes aperfluorinated ion exchange polymer in a volume percent between 35% and55%.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate embodiments of the present invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

FIG. 1 pictorially depicts the component parts of a fuel cell havingfunctions described herein.

FIG. 2 graphically depicts CO stripping voltammograms forplatinum-chromium/C and Pt/C catalysts to illustrate the low catalyticactivity of platinum-chromium/C for methanol oxidation relative to Pt/C.

FIG. 3 graphically depicts polarization plots representingelectro-oxidation of crossover methanol at platinum-chromium/C and Pt/Celectrodes to further demonstrate the relatively low catalytic activityof platinum-chromium/C for methanol oxidation.

FIG. 4 graphically compares the performance of two DMFCs, one with aplatinum-chromium/C cathode and the other with a Pt/C cathode.

FIG. 5 graphically compares the performance of three (3) DMFCs with MEAsusing platinum-chromium/C with one MEA using Pt/C to illustrateperformance improvement consistency.

FIG. 6 graphically compares the performance effect of Nafion® content inthe catalyst layer for platinum-chromium/C and for Pt/C.

DETAILED DESCRIPTION

In accordance with the present invention, a cathode catalyst of aplatinum-chromium alloy provides reduced catalytic activity for methanolreaction at a DMFC cathode while maintaining or increasing catalyticactivity for the oxygen reduction reaction at the cathode. The netpotential at the cathode from the methanol oxidation and oxygenreduction is increased over the net potential obtained from aconventional carbon supported Pt (Pt/C) cathode catalyst. Theplatinum-chromium alloy may be unsupported or carbon supported.

A suitable DMFC has the functional fuel cell structure shown in FIG. 1.It will be appreciated that each structural element shown in FIG. 1 canbe implemented in a variety of structures known to those skilled in theart, except as specifically described herein to incorporate theplatinum-chromium cathode catalyst.

As shown below, we have demonstrated that improved DMFC performance isobtained with a platinum-chromium cathode catalyst compared to a Pt/Ccathode catalyst. The exemplary platinum-chromium catalyst was acarbon-supported Pt₃C alloy. In these comparative experiments, theplatinum-chromium/C catalyst was about 44 wt % platinum-chromium and thePt/C catalyst was about 40 wt % Pt. The anode catalyst was a PtRu blackor PtRu/C (45 wt %). The anode and cathode catalysts were dispersed inappropriate amounts in water, with an added perfluorinated ion exchangepolymer for ionic conduction adjacent the catalysts (e.g., 5% Nafion®solution (1100 EW, Solution Technology, Inc., USA). Exemplary cathodeink compositions were 65 wt % Pt/C and 35 wt % Nafion® and 66 wt %Pt₃Cr/C and 34 wt % Nafion®; anode ink compositions were 85 wt % PtRuand 15 wt % Nation® or 70 wt % PtRu/C and 30 wt % Nafion®. MEAs wereprepared by painting the catalytic inks on membranes of Nation 117®. Thedesired catalyst loadings are less than about 1.0 mg cm⁻² so the cathodecatalyst inks were applied to obtain an experimental loading of about0.6 mg cm⁻¹. In all cases the geometric active area of the MEA was 5cm². TABLE 1 Preferred composition of catalyst layers Catalyst Catalyst(wt %) Nafion ® (wt %) PtRu black 85 15 PtRu/C 70 30 Pt/C 65 35 Pt₃Cr/C66 34

The reduced catalytic activity of platinum-chromium/C for methanoloxidation compared to Pt/C is demonstrated by the CO strippingvoltammograms shown in FIG. 2 for a Pt loading of 0.6 mg cm⁻² in bothcases. To obtain these results, DMFCs with platinum-chromium/C cathodecatalyst and with Pt/C cathode catalyst were operated in a “drivenmode”, with methanol being oxidized at the fuel cell cathode andhydrogen evolving at the fuel cell anode, which acted as acounter/quasi-reference electrode (a dynamic hydrogen electrode, DHE).CO produced during the methanol oxidation was adsorbed onto theelectrode surface and then stripped to determine the surface chargedensity as a measure of catalytic activity. The charge density forsurface CO stripping on the platinum-chromium/C cathode was 33 mC cm⁻²compared with ˜67 mC cm⁻² for the Pt/C cathode. These results clearlyindicate a reduced catalytic activity of platinum-chromium/C formethanol oxidation compared with Pt/C.

Another indication of different activity of methanol towardsplatinum-chromium/C and Pt/C was obtained in direct measurements ofmethanol crossover. In these experiments, the cells were again operatedin a driven mode, with methanol oxidized at the fuel cell cathode andhydrogen evolved at the fuel cell anode, thereby serving as a hydrogencounter/quasi-reference electrode. After stabilizing the cell at openvoltage, a single voltammetric scan was applied to the fuel cell cathodeand current response recorded, typically in the range of 0.1-0.6 V. Inaddition to allowing the magnitude of crossover to be directly examined,the activity of the catalyst towards methanol could also be determinedin such an experiment from the kinetic part of the current-potentialplots.

As shown by the plots in FIG. 3, the regular Pt/C cathode catalyst issignificantly more active towards methanol crossing through the Nafion117® membrane than the platinum-chromium/C catalyst. For example, at ananode potential of 0.35 V, a typical DMFC operating potential, thecurrent density of methanol oxidation with a platiunum-chromium/Ccatalyst is about 20 mA cm⁻², much lower than that of 81 mA cm⁻²obtained with the Pt/C catalyst. Not surprisingly, the differences inthe rate of methanol oxidation disappear once limiting-currentconditions are reached on both electrodes, i.e. at a potential higherthan 0.5 V. The same current density of ˜130 mA cm⁻² is measured ineither case, thus attesting to the expected similar permeation rates ofmethanol through the Nafion 117® membrane used with bothplatinum-chromium/C and Pt/C cathode catalysts. However, at lowerpotentials, the cathode using the platinum-chromium/C catalyst that isless sensitive to methanol is expected to remain at a higher netpotential than the Pt/C cathode, which is more susceptible to becomingdepolarized by methanol.

FIG. 4 graphically compares the performance of a DMFC with an exemplarycarbon-supported Pt₃Cr alloy (Pt₃Cr/C) cathode catalyst and a DMFC withPt/C cathode catalyst. The cathode catalyst loadings were 0.6 mg-cm⁻²,the anode catalyst loadings were 9.6 mg cm⁻² of PtRu black. The cellswere operated at 80° C., cathode pressure of 2.7 atm, with an anode feedof 0.5 M methanol. Except for the lowest current density range, belowabout 20 mA cm⁻², where performance of both catalyst composites isdominated by the high flux of methanol through the Nafion membrane, thePt₃Cr/C catalyst consistently showed a 70-80 mV voltage advantage overthe reference Pt/C catalyst. This suprisingly large voltage advantage ofthe Pt₃Cr/C catalyst has not been observed in hydrogen fuel cells,although a small voltage advantage is realized by Pt₃Cr/C because of ahigher activity of the catalyst in oxygen reduction at a cathode.

Relative performance of the two cathode catalysts was also tested versusanodes prepared by using carbon-supported PtRu catalyst at a low loadingof ˜1.0 mg cm⁻². Three different MEAs using Pt₃Cr/C cathode and PtRu/Canode were made to test reproducibility of the results obtained with thenovel cathode catalyst formulation. Hydrogen-air cell polarization plotswere then recorded as initial tests of cathode activity at a celltemperature of 80° C., showing very good and reproducible cathodeperformance. In particular, a cell current density of 0.2 A cm⁻² wasreached at 0.83-0.84 V, i.e., at a cell voltage similar to that measuredwith highly loaded (9.6 mg cm⁻²) unsupported PtRu anode.

Following operation in hydrogen-air mode, the above cells underwentregular DMFC testing. Polarization plots for the cells with Pt₃Cr/C andPt/C cathodes and PtRu/C anodes at 80° C. are shown in FIG. 5. All plotsrepresent MEAs having the same catalyst loading and operated under thesame fuel cell operating conditions. FIG. 5 shows again the verysignificant performance advantage of the platinum-chromium/C catalystover the reference Pt/C catalyst. As in the testing performed withhighly loaded PtRu anodes, the voltage advantage offered by Pt₃Cr/Ccatalyst over regular Pt/C catalyst with low PtRu/C anode loadings (˜1.0mg cm⁻²) was 60-80 mV in the entire range of investigated cell currentdensities. The results obtained with three different platinum-chromium/Ccells were highly reproducible, with current densities remaining withinjust a few percent from one another, as further shown in FIG. 5.

To further verify the repeatable nature of the results, differentpreparation batches (OMG) of the cathode catalyst and a catalyst fromanother manufacturer (E-TEK) were prepared and tested. Methanol-airpolarization plots indicate that there is no significant differencebetween these catalysts over the total current density region.

In the cathode catalyst layer, an optimal ion-exchange polymer contentshould minimize both ohmic and mass transport limitations, maximizeelectrochemical activity and Pt utilization. The influence of polymercontent in the catalyst layer with Pt/C or platinum-chromium/C onperformance of methanol/air fuel cell is shown in FIG. 6. An increase inNafion® content improves the performance up to 35% or 40% for Pt/C orplatinum-chromium/C, respectively, but platinum-chromium/C catalystshows a significantly greater performance improvement than does Pt/C.However, when too much polymer (beyond ca. 52%) is introduced thecurrent density at 0.45 V decreases because the ohmic and mass transportlimitations appear. This is a reasonable result because the ca. 52%Nafion® content means that the volume of Nafion® in the catalyst layeris almost the same as that of the catalyst. It is obvious thatplatinum-chromium/C is sensitive to the polymer content.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A direct methanol fuel cell (DMFC) having a methanol fuel supply,oxidant supply, an anode electrode and a cathode electrode with a solidpolymer electrolyte membrane therebetween forming a membrane electrodeassembly, a methanol oxidation catalyst adjacent the anode electrode andthe membrane, an oxidant reduction catalyst adjacent the cathodeelectrode and the membrane, wherein the improvement comprises an oxidantreduction catalyst layer of platinum-chromium alloy catalyst thatincludes a perfluorinated ion exchange polymer at about 35 to 55 volumepercent of the layer, so that oxidation at the cathode of methanol thatcrosses from the anode through the membrane to the cathode is reducedwith a concomitant increase of net electrical potential at the cathodeelectrode.
 2. The DMFC of claim 1, where the platinum-chromium catalystis provided at a loading less than about 1.0 mg cm⁻².
 3. (canceled) 4.(canceled)
 5. The DMFC of claim 1, wherein the platinum-chromium alloycatalyst is carbon-supported.
 6. The DMFC of claim 1, wherein theplatinum-chromium alloy catalyst is unsupported.
 7. (canceled)
 8. Amembrane electrode assembly (MEA), having an anode electrode, a cathodeelectrode with a solid polymer electrolyte membrane therebetween, foruse in a direct methanol fuel cell (DMFC), further including a methanoloxidation catalyst adjacent the anode electrode and the membrane, anoxidant reduction catalyst adjacent the cathode electrode and themembrane, wherein the improvement comprises an oxidant reductioncatalyst layer of platinum-chromium alloy that includes a perfluorinatedion exchange polymer at about 35 to 55 volume percent of the layer, sothat oxidation at the cathode of methanol that crosses from the anodethrough the membrane to the cathode is reduced with a concomitantincrease of net electrical potential at the cathode electrode.
 9. TheMEA of claim 8, where the platinum-chromium alloy is provided at aloading less than about 1.0 mg cm⁻².
 10. (canceled)
 11. (canceled) 12.The MEA of claim 8, wherein the platinum-chromium alloy catalyst iscarbon-supported.
 13. The MEA of claim 8, wherein the platinum-chromiumalloy catalyst is unsupported.
 14. (canceled)